Neurogenetic studies provided insight into molecular basis of tens of childhood neurological diseases but in the context of intellectual disabilities an autism spectrum disorders (ASDs) the challenge is how one can explain, similar phenotypes in face of vastly different molecular perturbations. This led to the proposal that the observed behaviors result from some shared patterns of circuit dysfunction. There is a pair of disorders with overlapping phenotypes that presents a unique opportunity to explore circuit-level disruption of homeostasis and its sequelae, particularly for learning and memory: Rett syndrome (RTT), caused by loss-of-function mutations in the X-linked MECP2, and MECP2 duplication syndrome, caused by duplications (or triplications) of the gene. The mechanism accounting for overlapping phenotypes in two syndromes with opposite molecular defects (and transcriptional alterations) remains mysterious. My preliminary data show that both loss and gain of MeCP2 function leads to increased cross-correlations of spontaneous calcium activity and increased sensitivity to GABA blockade in acutely isolated hippocampal slices. This hypersynchrony is caused by an imbalance of excitation and inhibition in the hippocampal circuit. The objective of this proposal is to characterize the malfunction of the hippocampal CA1 circuit in mouse models of RTT and MECP2 duplication syndrome and to explore the possibility of intervention and rescue. I hypothesize that loss and gain of MeCP2 function generates similar patterns of malfunction (circuitopathies) in canonical neural circuits, and that these circuitopathies can be rescued by restoring the imbalance of excitation and inhibition. To address this hypothesis, I propose three specific aims: (1) Elucidate the cellular mechanism underlying the malfunction of hippocampal CA1 circuit caused by MeCP2 dysfunction; (2) Determine how the circuit in hippocampal CA1 responds during learning tasks in Mecp2 mutants; (3) Determine whether altering cellular excitability acutely or chronically can rescue circuit malfunction in MeCP2 disorders. Given that circuit function is what eventually determines behavior, these studies will elucidate the circuit-level mechanism accounting for overlapping phenotypes of Rett syndrome and MECP2 duplication syndrome. It will also reveal the circuit's response to a hippocampal dependent learning behavior and uncover potential correlations between circuit malfunction and behavioral deficits. The data will provide insights into the value of manipulations at the circuit level and might lead to the design of new therapeutic approaches. Research proposed in the K99 mentored phase (year 1 and 2) is concentrated on generating multiple genetic mouse lines to elucidate the cellular mechanism of circuit malfunctions caused by Mecp2 mutation and establishing in vivo calcium imaging with a miniature head-mounted microscope on freely moving mice performing a learning task, all of which will be carried out with the supervision of Dr. Huda Y. Zoghbi and Dr. Stelios Smirnakis. During this phase, I will acquire training on new skills to directly prepare me for a career as an independent research scientist through scheduled weekly lab meetings, mentor supervision and discussion sessions, attending weekly departmental seminars, journal clubs and retreats, participation in joint lab and co- institutional meetings, attendance and participation at national scientific conferences, and enrollment in formal courses. The R00 Independent phase (year 3 to 5) will focus on determining how the circuit in hippocampal CA1 responds during learning tasks in Mecp2 mutants and explore the rescue of Mecp2 disorders. During this phase, I will continue an active relationship with my mentors through scheduled phone conversations and email. I will rely on their input and advice towards hiring lab personnel, budget development, and developing a successful R01 grant application.